I am not trying to be flip with my answer. You should to expect pay what you think is a fair price based on looking at and evaluating various guns that meet the criteria you are looking for. You will in relatively short order develop a sense of what a particular gauge, grade, and condition is going for depending on condition. These guns especially NH and EH 10's are well over 100 years old. They all generally have some flaws. Might be worn checkering, lack of barrel finish, cracked stock head, pitted barrel, off face, or a combination. Many of these things can be fixed so another trick is to know the cost of the repair and weigh that against the purchase price to figure where you'll be when the gun is "done".
Now you said, your criteria was shootable bores, which I take to mean some pitting and 50% and tight. I'll hazard an educated guess the range will be $800 - $1800 give or take. As condition goes up, so does the price. Here is an example of such a gun in that range, NH 10ga here:
http://www.stevecobbguns.com/amersg.htm
Good luck, another thing that can be hard to find in the old 10's is modern dimensions. Most of them were made in the era where shooters preferred a lot of drop.
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